For the convenience of the explanation of the present invention, a mobile phone will be taken as a typical example of the electronic devices. The mobile phone has a structure which generates a vibration to inform the user of an incoming call when it is set in the so-called “manner mode” of operation. Such a mobile phone incorporates a vibration generator as a vibration actuator which generates a vibration. The vibration generator of this type is disclosed in the Japanese Patent No. 3187029. The disclosed vibration generator is a small vibration motor having an eccentric weight intended for generation of a vibration.
However, the above-mentioned conventional vibration generator has problems as will be described below:
The motor with a brush is not reliable on the operation of vibration generation because the non-revolution problem caused by a so-called slit short-circuit cannot be eliminated.
Also, the motor body can be designed to have a diameter as small as about 3.5 mm, but the motor speed and power consumption are too great. Apparently, the power consumption should be as small as possible because a smaller power consumption will lead to a longer service life of the battery used in a portable electronic device such as the mobile phone. Along with the demand for an smaller and thinner design of the electronic device in which such a vibration generator is to be incorporated, it has been demanded to design the smaller and simpler vibration generator and the electronic device which is to use the vibration generator in it.
To meet the above requirements, it has been proposed to design the vibration generation like a small coin. For designing the flat vibration generator of this type to be more compact, the rotor is reduced in weight due to the smaller and thinner design with such a result that the friction will be increased because the attraction of the rotor toward a bottom plate by the magnetism of a magnet increases. On this account, the magnet may be designed smaller or the bottom plate is formed thinner to reduce the magnetism. However, this will lead to a reduction in running torque of the rotor and also to an insufficient structural strength. It will be difficult to provide a highly reliable vibration generator.